Chapter 22Biogeochemical cycling

The Universe

When? How? 15 X 109 years ago

Matter existed in its most fundamental form.

Elements formed as universe expanded and cooled.

13.8 sec post ‘Big Bang’ –formation of H and He nuclei

700,000 years later—electrons attached to nuclei

Formation of elements

Elemental formation is linked to evolution of stars.

Stars derive energy from nuclear reactions that synthesize elements.

4He + 4He 8Be 8Be + 4He 12C 1H + 4He 5Li 12C + 4He 16O

Ecosystems are linked

Input and output of nutrients link ecosystems

Gaseous cycle—

atmosphere and ocean

Sedimentary cycle—

soil, rocks and minerals

dissolved salts and rock phase

Basic element of all organic compounds

Inseparable with energy flow

Source of CO2

atmosphere/water

Primary producers decomposers

Carbon

Net ecosystem productivityRate at which C is taken up in

photosynthesis and lost due to respiration.

Determined by Primary productionDecomposition

Terrestrial ecosystems—slower in cooler climates—slower decomposition and production

Aquatic C cycling

Phytoplankton uses CO2 or carbonate

CO2 enters back into system through respiration and decomposition

Variation in C cycling

Varies with time of day—photosynthesis highest in afternoonrespiration highest just before daylight

Seasonal variation—varies according to weathervaries with climatevaries with seasonal

More pronounced in terrestrial ecosystems

Carbon stores

1023 grams of C = 100 million Gt (1 Gt = 109 tons)

55,000 Gt in C poolOceans –38,000 Gt

dead organic matter –1500 Gtliving biomass – 750 Gt

Terrestrial – dead organic matter – 1500 Gt living biomass – 560 GtAtmosphere – 750 Gt

Carbon exchange

Ocean exchange site — surface water

Circulates via currents and movement through food chain

Terrestrial exchange site – governed by photosynthesis / respiration

Large stores in soil

increases from tropics poleward

Nitrogen cycleEssential in proteins rubisco

Usable forms = NH4+ and NO3

-

N stores in atmosphere = N2

N enters ecosystem through:

wetfall/dryfall

N fixation

Cosmic radiation/lightening/meteor trails

biologically— N fixing bacteria

Biological N fixation

Provides 90% of available N to ecosystems

Splits N2 into 2 N + H+ NH3

For each gram NH3 use 10 grams glucoseAgents

Legumes/symbiotic bacteriafree-living aerobic/anaerobic bacteria—Azotobacter/ClostridiumCyanobacteria (blue-green algae)—Nostoc/CalothrixLichens

N availability

Ammonification –process of breaking down organic matter and producing NH3

Soils slightly acidic

Quickly converts to NH4+

Nitrification –converting NH4+ to NO2

- and then to NO3

-

Denitrification—reduction of NO3

- to N2O and N2

N export & stores

NO3- most common form exported

High demand for Necosystem and global cycling similarN stores Atmosphere –largest pool 3.9 X 1021

Biomass and soils – 3.5 X 1015 / 95-140 X 1015

Oceans—inputs from rivers and atmosphere 36 X 1012 / 30 X 1012

Biomass—15 X 1012

Denitrification returns 110 X 1012 to atmosphere

Phosphorus cycle

No atmospheric input--follows hydrological cycle only

Often in short supplyReservoirs – Rock + natural phosphate

depositsInternal cycling important—3 states

organic P, dissolved organic P & inorganic PInorganic P taken up by primary producers

eaten by zooplankton—excreted or retainedP used by bacteria not recycled

P can be deposited into sediments

Global cycling unique—no atmospheric inputs /

river inputs important in oceansHigh turnover rate

Sulfur cycle

Sedimentary and gaseous phasesCarried in salt solutions

tied up in deposits—released by weatheringAtmospheric input—fossil fuels, volcanic

eruption, ocean surface water, decomposition

Enters as H2S—oxidized to SO2 carried as H2SO4 –result = acid rainImportant in amino acids

Decomposition—released as HSO4- or SO4

2-

Presence of Fe, S precipitates out as FeS2

Global cycling of SLeast understood of nutrientsGas phase allows global cyclingInputs:Oceans contain large pools, but do not contribute much Input into atmosphere:Forest fires Volcanic Industrial

Oxygen cycleComplex cycle—linked to other nutrients

Sources of O2 photosynthesis

breakup of H2O in atmospherePresently --balance of

photosynthesis and respiration

O2 produced as byproduct of anaerobic respiration

O2 released by weathering of rocksO available in water and carbon dioxide

Redfield ratio

Cycles of nutrients are linkedStoichiometry—quantitative relationships of

elements in combinationRedfield ratio—constant atomic ratio despite

ambient nutrient concentrations C:N:P 106:16:1

106CO2 + 16NO3- + HPO4

2- + 122 H2O + 18H+ (CH2O)106(NH3)16(H3PO4) + 139 O2